It is known to provide a staircase which comprises at least one rigid stringer and a plurality of horizontal treads spaced apart along the stringer with a predetermined tread rise, i.e. vertical distance between the upper surface of one tread and the upper surface of the tread thereabove, and predetermined tread depth, i.e. horizontal distance between the projecting edge of the tread and the rear thereof. The stringer can be a single piece or can be formed by interconnected and detachable or permanently fixed pieces and, moreover, can be fabricated by connecting horizontal supports and vertical supports so that they can be disassembled or are permanently secured to one another.
A staircase of this type is described in German Auslegeschrift (published application (U.S. Pat. No. 1,659,756 (U.S. Pat. No. 3,474,882) and uses rigid "Z" shaped supports which are joined by inserting one vertical leg of a first Z support into a vertical leg of another Z support and either allowing the connection to pivot or locking the connection together. A sleeve and pin system or a telescoping sleeve arrangement can used to connect the Z shaped supports.
While this type of staircase has the advantages that it can be used for different staircase configurations, e.g. as a straight staircase, a staircase with one or more landings or bends, or a spiral staircase, it has the disadvantage that, for a given distance between two floors to be bridged by the staircase, the latter must extend over a fairly large base area. The term "base area" is used to describe the area occupied by the staircase as seen in plan view upon the supporting surface, i.e. the lower floor to which the staircase extends downwardly.
In normal circumstances, such as staircase may have twelve to fourteen treads or steps and, because the tread must have a given width (tread depth ignoring any overhang), the total base area or floor area required for the staircase can be obtained by multiplying the tread width by the number of treads forming the staircase. Where sufficient area is available, no problems arise. Frequently, however, there is insufficient area available fot the staircase and, accordingly, the staircase must be steeper than is comfortable or convenient. If the same number of treads must be used because of the given rise between treads, the treads overhanging one another quite substantially make it practically impossible to negotiate the staircase without striking the top of ones tow upon an overhanging tread in ascent and without obtaining purchase of the heel portion of the foot upon descent. Of course, if the tread width is narrowed substantially a sufficient portion cannot be obtained in either ascent or descent.
As a consequence, the use of the staircase frequently involves danger to the user.